Kangaroos have an aerobic capacity comparable to highly athletic mammals like horses and antelopes, new research shows, which may dent their reputation as laid-back fixtures on the Australian landscape.

Marsupials, including kangaroos, have traditionally been believed to be primitive mammals because their basal metabolism, or resting energy rate, is about a third lower than that of placental mammals.

But Emeritus Professor Terry Dawson from the University of New South Wales in Sydney says analysis of the muscle physiology of kangaroos shows they stack up against their more energetic placental cousins.

Dawson studied the muscles of the red kangaroo Macropus rufus, specifically the muscles' mitochondria, the organelles responsible for producing energy.

The more mitochondria in muscles, the more energy available.

"What we've shown is that kangaroos have as much total mitochondria in their muscles as the most athletic of placental mammals," Dawson said.

Marsupials, and in particular kangaroos, may even be more efficient than placental mammals because of their ability to "rev up" very quickly.

"Instead of being primitive they've got the benefits of not using much energy when they're resting, but they have the capacity to really rev themselves up," he said.

"We've probably shown that instead of being primitive they probably have advantages in terms of energy uses."

Clues to evolution

Dawson said the research also pointed to new evolutionary patterns.

Marsupials and placentals, which comprise the theria or "advanced mammals", diverged in the early Cretaceous period about 125 million years ago.

It was previously believed the placentals subsequently evolved a highly developed aerobic system.

Dawson said similarities in energy delivery systems between marsupials and placentals, including mitochondria, musculature and cardiovascular system, suggested the evolution of aerobic capacity occurred before the split.

"There's some suggestion that mammal-like reptiles around during early stage of dinosaurs had developed this highly aerobic capacity," he said.

Dawson said studying aerobic capacity in marsupials could help sports scientists improve athletic performance in humans.

Unlike kangaroos, which have very little energy production in their forelegs, humans can work with both their upper and lower limbs.